To install a hollow fiber membrane in a water filtration plant quickly and readily, the hollow fiber membrane is generally provided in a form of a module. The hollow fiber membrane module includes multiple strands or bundles of hollow fiber membranes and a manifold of various shapes, and the manifold functions to support and fix the multiple strands or bundles of hollow fiber membranes, and to collect treated water permeating pores of the hollow fiber membranes.
Examples of the hollow fiber membrane module are disclosed in Japanese Patent Publication No. 7-155564, Japanese Patent Publication No. 7-178321, and Korean Patent Publication No. 10-2000-0050208, and an example of the cartridge-type hollow fiber membrane module easily detachable from a holding frame for the hollow fiber membrane module is disclosed in U.S. Pat. No. 6,325,928, and as known in the previous development process, objectives to take notice of in design of the hollow fiber membrane module are integrity, easiness in mounting and detaching, and easy cleaning.
The term “integrity” refers to providing a maximized surface area of hollow fiber membranes in an installation space of the same volume, the term “easiness in mounting and detaching” refers to minimization of a troublesome hand work needed to mount/detach the hollow fiber membrane module in/from the holding frame for the hollow fiber membrane module, and the term “easy cleaning” refers to a simple process of removing materials clogged between the membranes of the hollow fiber membrane module occurred during operation of a water filtration plant.
With the advancement of hollow fiber membrane modules, there were considerable improvements in integrity, easiness in mounting and detaching, and easy cleaning, but there is still room for improvement. Particularly, in the aspect of easy cleaning, more improvements are required, and in the case of filtration involving immersing a hollow fiber membrane module in wastewater with a high concentration of floating matters, there is a phenomenon, called intermembrane clogging, in which floating matters accumulate on the hollow fiber membrane surface and between hollow fiber membranes, resulting in a reduction in effective surface area of the hollow fiber membranes for use in water permeation. In the event of intermembrane clogging, a water permeation rate remarkably reduces and a service life of the hollow fiber membrane module decreases, and to alleviate the intermembrane clogging phenomenon, it is general to continuously remove floating matters accumulated on the hollow fiber membrane surface and between the hollow fiber membranes by shaking the hollow fiber membranes through air aeration, turbulence formation, and generation of ultrasonic waves or vibration from the bottom of the hollow fiber membrane module during operation of the water filtration plant. However, in spite of this solution, intermembrane clogging goes beyond an allowable level after a long-term operation.
Let us see a structure of a traditional general submerged hollow fiber membrane module. A hollow fiber membrane is fixed at both ends. Although air aeration or turbulence is formed by an air diffuser apparatus, there is a limitation on movement of the hollow fiber membrane. Despite a cross flow of treated water or a cleaning process by supplied air, it is not easy to remove contaminants accumulated on the hollow fiber membrane surface, and because the air diffuser apparatus is installed below the module, cleaning air is not directly transferred to the hollow fiber membrane module, so an air loss is large and a high rate of air aeration is additionally required, resulting in a high operation cost.
Dissimilar to the membrane module having the hollow fiber fixed at both ends, a submerged hollow fiber membrane with a free end that is fixed at one end and not fixed at the other end is typically disclosed in Japanese Patent No. JP3130997 by Mitsubishi Rayon and Japanese Patent Publication No. 11-128692 by Toray, and one of the problems that are usually found in a module including the hollow fiber membrane fixed only at one end is entanglement of the hollow fiber membrane due to an abnormal flow in a fluid flow of treated water generated by the collapse of the hollow fiber membrane in the module or different amounts of air during operation, and when mechanical stresses caused by the abnormal flow concentrate at the tangled part, the hollow fiber membrane may be cut off.
More specifically, the collapse of the hollow fiber membrane in the module often occurs when taking the hollow fiber membrane module out of an aeration tank for the purpose of cleaning or maintenance and repair, and a fluid flow goes from a region with a larger amount of air supply to a region with a smaller amount of air supply during operation, and along the flow, the hollow fiber membrane has an orientation, thus inducing entanglement, or when the hollow fiber membrane module gets inclined at any one side, a contact location of a flow of supplied air and the module differs, which fails to induce a fluid flow of a predetermined pattern, resulting in entanglement, and in the end, cut-off of the hollow fiber membrane.
These related arts have the following disadvantages; a manufacturing cost increases due to installation of a support for preventing the collapse caused by a long length of the hollow fiber membrane, damage occurs in the hollow fiber membrane by friction between the support installed additionally and the hollow fiber membrane with flowability, and contaminants accumulate near an additional mechanical structure, and it is difficult to remove the contaminants. Furthermore, due to the characteristics of the long hollow fiber membrane and the module, work efficiency in constructing a water treatment facility or installing the module reduces, and when a hollow fiber membrane is contaminated during use, it is impossible to remove contaminants after and re-installation in a simple and convenient manner of easily taking out a cartridge, cleaning through high pressure water cleaning, and re-installing the cartridge, so the convenience in maintenance and repair for solving problems that may occur during operation is low.
It is known that a submerged water treatment system needs an air cleaning method using bubbles as a solution to a high level of membrane fouling. However, the air cleaning method using bubbles has a drawback of enormous energy consumption as noted, and studies have been made to increase efficiency of an air diffuser apparatus.
To provide gas enough to effectively clean the membrane surface with reduced energy consumption, a cyclic aeration system designed to provide bubbles periodically is required, and to implement the periodic aeration operation, generally a complex valve array and a control means are needed, which tends to increase an initial system cost for the complex valve and switching array and a subsequent maintenance and repair cost. Also, within a large scale system, the number of circulations is limited by a functional constraint of a mechanical valve. Thus, it is preferred to provide a method for energy efficient cleaning while not requiring to control the operation by means of complex valve switching.
To overcome this limitation, an air aeration method using an airlift pump has been developed, and its related art is disclosed Korean Patent Publication No. 2010-0023920 by Siemens directed to membrane cleaning using an airlift pump and Korean Patent Publication No. 2010-0012844 and Korean Patent Publication No. 2012-0093402 directed to a method and apparatus for membrane cleaning using a pulsed airlift pump, but these methods have a limitation on arrangement and shape of a membrane module due to creation of macro bubbles through an intermittent or pulsed fluid flow irregularly generating along the membrane surface to remove contaminants from the membrane surface, and allow intermittent aeration only.
Accordingly, there is a need for technology development of an air diffuser apparatus that may allow for intermittent aeration to reduce an amount of cleaning air, allow for various arrangements based on arrangement and shape of a membrane module, and variably control the continuous aeration operation based on changes in raw water and a concentration of solids in a membrane separation tank, and a water treatment method using the same.